Four-wave mixing (FWM) is an intermodulation phenomenon where at least two different frequency components propagate together in an optical waveguide, such as an optical fiber, to generate an idler emission having a longer wavelength than the wavelengths of the frequency components propagating in the waveguide. Further, one of the pre-existing frequency components can be amplified through parametric amplification of the different frequency components propagating in the optical waveguide. FWM is often applied for spectroscopy where two input waves generate an idler emission with a slightly higher optical frequency. FWM can also be applied for phase conjugation, holographic imaging, and optical imaging processing.
Degenerate FWM results when one or more pump lasers produce the same frequency component (i.e., a single pump laser produces a laser beam at one frequency or two pump lasers produce laser beams having the same frequency), and the frequency from the one or more pump lasers is used to provide amplification to a signal laser. The signal laser is often referred to as a seed laser because it helps initiate the FWM process and is typically a lower powered laser as compared to the one or more pump lasers.
During FWM, the pump laser provides the energy to generate the idler emission while also amplifying the frequency component of the signal laser. More specifically, for every photon being added to the signal frequency component, two photons are taken away from the pump frequency component with the remaining photon being added to the idler emission. As FWM is a phase-sensitive process, its effect can efficiently accumulate over longer distances if a phase matching condition is optimized. On the other hand, if there is a strong phase mismatch, FWM is effectively suppressed.
So long as the phase matching condition is satisfied, the frequency component of the idler emission is a function of various operating conditions including primarily the pump laser(s)′ wavelength and power, the signal laser's wavelength and power, and transmission and dispersion properties of the optical waveguide where the FWM occurs. Given that the pump laser(s)′ wavelength is typically constant based on the pump laser(s) selected, limitations in the spectral tuning range during FWM can be encountered due to the transmission and dispersion properties of the optical waveguide. Hollow core photonic crystal fibers (HCPCFs) filled with a noble gas have been found to be an ideal optical wave guide for optimizing the FWM phase matching conditions and for the avoidance of Raman interactions during FWM.
What is needed is a system for optimizing FWM efficiency in gas-filled HCPCFs by making sure the phase matching condition is met by the pump and signal frequency components over a wide spectral bandwidth for the idler emission.